Paper coated with polylactide and a method for making it

ABSTRACT

The invention relates to a polylactide-coated paper or board product which is made by coextrusion wherein the polylactide is extruded together with a conventional polymer, such as polyolefin, onto the surface of the paper, and after extrusion the polyolefin film is removed and recycled.

This application is the national phase under 35 U.S.C. §371 of prior PCTInternational Application No. PCT/FI97/00293 which has an Internationalfiling date of May 20, 1997 which designated the United States ofAmerica.

The invention relates to paper and board products which are coated withpolylactide and to a method for the manufacture of such products.

The coating of paper and board with various polymers or waxes is acommonly used method. The purpose of the coating is to improveresistance to water and other barrier properties, sealability, strengthproperties and gloss. The coating is typically done with polyolefins, inparticular polyethylene, or by using multilayer structures.

One problem involved with coated paper and board is their poorrecyclability. Polyethylene, which is commonly used, does not degrade inthe conditions used in the repulping of paper. Also, polyethylene-coatedpaper does not break down completely in nature and does not compost.When the aim is biodegradable products or recycling, the paper must becoated with a biodegradable plastic, such as polylactide.

The extrusion coating technique is typically used in the coating ofpaper; high temperatures and high run speeds are usually employed inthis technique, and additionally a thin but tightly adhering plasticlayer is desired on the paper or board.

The use of polylactide for the coating of paper has been describedpreviously in patent publication WO094/08090. The said publicationdiscloses the coating of paper with a chloroform solution which contains20% of polylactide. Alternatively with molten polylactide, which isadded by means of a nozzle very close to the paper surface. By thismethod it is possible to prepare only a coating which is relativelythick, more than 25 μm. If the distance of the nozzle from the paper isincreased, the molten polylactide is not spread evenly. This is due tothe fact that the melt strength of the polymer is not sufficient for theformation of a proper film.

A typical method for the coating of paper is to extrude molten polymerthrough a nozzle onto a moving paper web. The paper may be treated(activated) by a separate corona or plasma treatment, or even byheating. The distance of the nozzle from the paper is regulated, forexample, according to the polymer. The distance affects, for example,neck-in. However, neck-in is also affected by the stabilization of thepolymer and by additives; neck-in is greater for an unstabilized polymerthan for a stabilized polymer.

The biggest problem in coating with polylactide is the rapid cooling ofthe polylactide film after it emerges from the nozzle before it ends upon the paper, even if the nozzle is kept as close to the paper aspossible. This results in poor mechanical adhesion.

However, it has now been observed, surprisingly, that too rapid coolingof the polylactide can be inhibited by coextrusion, wherein thepolylactide is extruded together with some polymer conventionally usedfor coating, such as polyolefin, in such a manner that the polylactidelayer will be against the paper and the polyolefin layer will betopmost. It is preferable to use, for example, polyethylene having amelt index of approx. 5-20 g/10 min. An excellent adhesion of thepolylactide film to paper is achieved by this method. After theextrusion, the polyolefin film, which detaches easily from thepolylactide coating, is removed before the reeling of the coated paperand is recycled. Thus a compostable, completely biodegradable or easilyrecyclable paper or board product is obtained.

The difference of polylactide from polyethylene, which is normally usedfor coating, is its narrower processing window, i.e. lower resistance tohigh temperatures and more rapid cooling. Characteristics typical ofpolylactide, on the basis of which adhesion would be very good, are itslower viscosity and high polarity. Low viscosity will cause goodmechanical adhesion through spreading onto the paper and into its pores.Polarity causes dipole--dipole interactions with normally polar paperfibers. The oxygen and gas barrier properties of polylactide are betterthan those of polyethylene or polypropylene, but its water vapor barrierproperties are poorer. The sealability of polylactide is very good.

Viscosity can be further lowered by means of plasticizing agents. Theagents used must, however, be biodegradable and approved for contactwith food.

Polylactide has a higher surface energy than typical polyethylene andpolypropylene films.

The surface energy of polyolefins is normally 30-33 mN/m, for whichreason they have to be treated to improve printability. The surfaceenergy of polylactide is approx. 40-44 mN/m, and thus treatments are notnecessary; the coated paper and board products are easily printable.

Lactic acid, the principal degradation product of polylactides, i.e.condensation polymers based on lactic acid, is a product common innature; it is not toxic and is used widely in the food andpharmaceutical industries. The high molecular weight polymer can beproduced by ring-opening polymerization from lactic acid dimer, lactide.Lactic acid is optically active, and thus its dimer appears in fourdifferent forms: L,L-lactide; D,D-lactide; L,D-lactide (mesolactide);and a racemic mixture of L,L- and D,D-lactides. By polymerizing theseeither as pure compounds or in various blend proportions, polymers areobtained which have different stereochemical structures affecting theirresilience and crystallinity and, consequently, also their mechanicaland thermal properties.

Upon forming, polylactide is in equilibrium with its monomer, lactide.This has sometimes been deemed to be advantageous, since monomers andoligomers may act as plasticizers of the polymer, but it also leads torapid hydrolysis and causes problems of adhesion in the processing ofthe polymer. Furthermore, the presence of the monomer lowers thermalstability during melt processing. In general the residual lactide mustbe removed from the polymer. The lactide content of the polylactide usedin the invention is below 5%, preferably below 2%.

The breaking down of polymers during processing can be reduced by theremoval of the residual lactide, the maintenance of the water content ata low level (below 200 ppm) or by the addition of commercialstabilizers. In terms of melt processing methods, it is, however, anadvantageous method to mix certain peroxides with the polymer, in whichcase thermal stability remains good and the melt strength of the polymeris improved so as to be sufficient for extrusion (FI935964, F1945264,Neste).

The polylactide used in the invention can be made from L-, D- orD,L-lactide, or blends thereof, by any polymerization process.Copolymers or polymer blends may also be used, but this is by no meansnecessary for the functioning of the invention. The use ofpoly-L-lactide is especially advantageous. The weight-average molecularweight (M_(w)) of the polymer according to the invention is approx.50,000-2,000,000.

Also usable are polymers which are prepared by first polymerizing a lowmolecular weight oligomer of lactic acid and by linking such oligomersto each other by either urethane or epoxy bonds, as disclosed in patentapplications FI924699, FI943250, FI951638 and FI952030.

A polylactide coating can be tailored effectively according to theintended use by the selection of a suitable plasticizer and, whenneeded, a filler.

Plasticizers and, when so desired, fillers and other additives, aremixed with the polylactide, before extrusion coating, by a conventionalmelt mixing method, for example in a double- or single-screw extruder orin a batch mixer.

As noted in patent applications FI935964 and FI945264, in polymerstabilization it is possible to use many even commercially availableorganic peroxide compounds, in particular those from which acids areformed as degradation products. Peroxides acting as stabilizers arecharacterized by a short half-life, preferably below 10 s, but mostpreferably below 5 s. Examples which can be given of suitable peroxidesinclude dilauroyl peroxide (half-life at 200° C. 0.057 s),tert-butylperoxydiethylacetate (0.452 s), t-butylperoxy-2-ethylhexanoate(0.278 s), tert-butylperoxyisobutyrate (0.463 s) andtert-butylperoxyacetate (3.9 s), tert-butylperoxybenzoate (4.47 s) anddibenzoylperoxide (0.742 s). The amount of peroxide to be used isapprox. 0.05-3% by weight. The required amount depends on the peroxidecompound and above all on the desired end product.

The products according to the invention can be used in the manner ofconventional coated paper and board products, in particular inapplications which aim at minimizing the amounts of waste and/or atprocessing waste by, for example, composting. This involves inparticular various packaging materials, also food packages, as well asdisposable plates and cups.

Normal paper and board grades intended for paper coating can be used inproducts according to the invention. Coextrudable polyolefins are alsoconventional polyethylene or polypropylene grades suitable for extrusioncoating, or copolymer grades thereof.

The invention is described in greater detail with the help of thefollowing examples.

The polylactide used in the experiments was made by ring-openingpolymerization from L-lactide with the aid of a stannium octoatecatalyst, and it was manufactured by Neste Oy. All of the polylactidesused in the examples were stabilized by peroxide in the manner describedin patent applications FI935964 and FI945264.

EXAMPLE 1. PLASTICIZED POLYLACTIDE Rheometer measurements

Processing temperatures were sought by means of rheometric measurements.The measurements were carried out using a Goettfert Rheograph 2002rheometer. The reference values used were the viscosity curves, measuredat 300° C., of two different polyethylene grades, the melt indices ofwhich were 4.5 and 15 g/10 min. It was noted that the viscosity ofpolylactide corresponded to the reference values at 245° C., whereas theviscosity curves of the plasticized samples corresponded to thereference values already at 230-240° C. The viscosity curves of theplasticized samples as compared with the viscosity curves ofpolyethylene and pure polylactide are shown in FIG. 1.

All of the polylactide coating experiments were carried out usingWisapak Oy's laboratory-scale extrusion coating machine, which has threeextruders and a nozzle having a width of 270 mm. The distance betweenthe paper and the nozzle was 15-80 mm.

The temperatures of the extruder zones ranged from 150 to 200° C. It wasnot possible to raise the temperatures to the optimal level (230-240°C.) determined in the rheometric measurements without losing the meltstrength of the polymer. Two extruders, having rotation speeds of 80 and260 rpm, were used simultaneously in the experiments. The speed of thecoating line was 10-50 m/min.

                  TABLE 1                                                         ______________________________________                                        Effect of plasticizers on the properties of polylactide                                Amount   M.sub.w        MFR.sub.2                                                                            Surface                                 p- % g/mol D (g/10 min) energy                                              ______________________________________                                        --        0       129000   2.4   1.2    40                                      Citroflex A-4 15 125000 2.2 4.1 40                                            Triacetin 15 121000 2.1 4.1 39                                                Tripropionin 15 122000 2.1 4.6 42                                             Santicizer 431 15 123000 2.0 3.1 42                                           Santicizer 160 15 131000 2.5 6.8 n.d.                                         Polyethylene 15 2200000  34 12.2 n.d                                          glycol                                                                        Lactid acid 10 125500 2.3 9.9 43                                              oligomer                                                                    ______________________________________                                    

By using plasticized samples, very thin (2-9 g/m²) and resilient coatingfilms were obtained at the coating speeds used. The films were even andglossy. All of the plasticizers except TA and CF A-4 raised the surfaceenergy. The nozzle neck-ins ranged from 26 to 38%. The adhesion of allof these thin films to paper was poor.

EXAMPLE 2. PEROXIDE-MODIFIED POLYLACTIDE

More peroxide was added by extruder mixing to basically stabilizedpolylactide. The mixing was carried out at 180-200° C. Owing to theperoxide modification, more branching was obtained in the polylactide,its molar mass distribution widened and molar mass increased, itspolydispersity increased, and its melt index decreased.

                  TABLE 2                                                         ______________________________________                                        Effect of peroxide modification on the properties of polylactide.                  TxC content                                                                             M.sub.w          MFR.sub.2                                       wt. % g/mol D g/10 min                                                      ______________________________________                                        0.1        120000       2.2   1.4                                               0.2 146000 2.4 0.8                                                            0.5 207000 3.4 0.5                                                          ______________________________________                                    

Owing to the peroxide modification, it was possible to use temperaturesof 230-240° C. in extrusion coating. Two extruders, having rotationspeeds of 90 and 130 rpm, were used in the trial runs. The coating speedwas 10-50 m/min. The coating film obtained was clear, had a basis weightof 9-55 g/m², and its adhesion to paper was moderate, but notsufficient.

EXAMPLE 3. EFFECT OF THE THICKNESS OF THE COATING FILM ON ADHESION

The effect of the thickness of the coating film on adhesion wasinvestigated by increasing the coating speed, whereby the thickness ofthe coating film was reduced. Two extruders were used in the trial run,their temperatures being 160-220° C. and their rotation speeds 230 and60 rpm. Preheating of the paper was experimented with by using a thermalpistol. Adhesion was measured by peel strength measurement (by applyingthe standard ASTM D 1876). It was observed that adhesion decreasedlinearly as the coating speed increased, i.e. as the coating film becamethinner. It was observed that preheating of the paper improved adhesion.FIG. 2 shows the peel strength values at coating speeds of 20-50 m/min.

EXAMPLE 4. FILLED POLYLACTIDE

When paper is coated with polyethylene, fillers are commonly used inorder for the polymer to remain hot longer. The use of fillers also withpolylactide was investigated. 10-15% by weight of talc was used as apolymer filler material. The extruder temperatures used were 215-220°C., the rotation speeds were 150 and 250 rpm. The coating speeds were10-40 m/min. The coating film obtained was gray, matte-surfaced andthin, having a basis weight of 4-16 g/m². The adhesion of the coatingfilm to the paper was poor, although theoretically the filler shouldkeep the polymer temperature longer at a high level, in which case thepolymer would form more mechanical bonds with the paper. In this case,however, the talc has acted mainly as an anti-blocking agent.

EXAMPLE 5. COEXTRUSION OF POLYLACTIDE

Coextrusion was carried out as a coextrusion of polylactide and apolyethylene having a melt index of 15 g/10 min, in such a manner thatthere was polylactide in two extruders and polyethylene in one, thepolyethylene film being topmost in relation to the paper. The processingtemperatures used were in the extruder 240° C. for polylactide and 275°C. for polyethylene and in the nozzle 255° C. The coating speeds usedwere 10-50 m/min and the extruder rotation speed was 200 rpm for each.The nozzle was kept at a higher level above the web being coated than inthe previous examples, owing to the low neck-in of polyethylene film.The polyethylene film was easy to detach from the obtained coating filmsbefore the reeling of the paper, whereas the adhesion of the polylactidefilm to the paper was excellent. When the coating speed increased above30 m/min, the polylactide layer became so thin that it no longerdetached easily from the polyethylene film. When the screw rotationspeeds were lowered to 80-170 rpm, a thickness as small as 8 g/m² wasobtained for the polylactide film while the polyethylene film was stillvery easily detachable.

As can be observed from the examples above, a good film can be extrudedin all cases from peroxide-stabilized polylactides, but owing, to toorapid cooling it will not as such adhere sufficiently well to a paper orboard base. By the use of the coextrusion method according to theinvention, a coated paper is obtained which has an even, well-adheringpolylactide coating.

What is claimed is:
 1. A polylactide-coated paper or board product,which is prepared by coextrusion in which the polylactide is extrudedtogether with a polyolefin onto the paper surface in such a manner thatthe polylactide is disposed adjacent to the paper and the polyolefin isoutermost, wherein the polylactide consists of a peroxide stabilizedL-polylactide (PLLA) and its monomer content is below 5% by weight, andafter the extrusion the polyolefin is removed.
 2. A paper or boardproduct according to claim 1, wherein the polylactide is coextruded withpolyethylene having a melt index 5-20 g/10 min.
 3. A paper or boardproduct according to claims 1 or 2, wherein one or more fillers, in anamount of 0.1-50% by weight, is mixed with the polylactide.
 4. A paperor board product according to claim 3, wherein said one or more fillerscomprises talc.
 5. A paper or board product according to claim 1,wherein a plasticizer in an amount of 0.5-30% by weight is mixed withthe polylactide.
 6. A polyactide-coated paper or board product as inclaim 1, wherein said polyolefin is recycled after removal from saidpaper or board product.
 7. A method to produce a polylactide-coatedpaper or board product, comprising the steps of:coextruding apolylactide and a polyolefin; coating said polylactide and polyolefinonto the surface of a paper so as to form a film on the surface of thepaper; removing the polyolefin from said paper; reeling said paper; andrecycling said polyolefin, wherein said film is made up of a firstportion in contact with the paper and a second portion not in contactwith said paper and said first portion is comprised on polylactide andsaid second portion is comprised of said polyolefin, wherein saidpolylactide consists of a peroxide stabilized L-polylactide with amonomer content below 5% by weight.
 8. A polyactide-coated paper orboard product with a film comprising at least two layers on top of saidpaper or board product, wherein a first layer comprises a polylactideadjacent to said paper or board product and a second layer comprises apolymer is on top of said polyactide, wherein the polylactide consistsof a peroxide stabilized L-polyactide (PLLA) with a monomer contentbelow 5% weight.
 9. A L-polylactide-coated paper or board product, whichis prepared by coextrusion in which the L-polylactide is extrudedtogether with a polyolefin onto the paper surface in such a manner thatthe L-polylactide is disposed adjacent to the paper and the polyolefinis outermost, wherein the L-polylactide consists of a peroxidestabilized L-polylactide (PLLA) and its monomer content is below 5% byweight, and after the extrusion the polyolefin is removed.